This invention relates to web break detector systems in general and more specifically to web break detection systems utilizing photoelectric principles.
Optical scanning systems have for many years been used to monitor the condition of moving webs of paper, cloth and the like in processing mills and printing presses. The object of such devices is to rapidly detect tears and other discontinuities in the web, and in particular along the edges of the web. Such devices then shut down or otherwise disable the equipment before the break can increase in size and accumulate the running material in or around the many rollers and other operating mechanisms of the press. In view of the extremely high speed at which many webs travel, a minor tear can cause a major equipment breakdown when all or a portion of the web begins to accumulate within the system. Typically, therefore, web break detectors are used not only to shut down systems but to energize automatic web cutting blades to immediately stop the flow of the web through the equipment.
In optical web guide detectors, light sources have been used that emit in the infrared region. Filtered detectors have been employed to detect the infrared emission while being generally nonresponsive to the light environment outside of the infrared spectrum. A system of this type incorporating a synchronized and pulsating type infrared detection system is shown in U.S. Pat. No. 3,906,232 of Meihofer. As noted therein, early photoelectric web break systems positioned the receiver on the opposite side of the web from the emitter. However, due to the desirability of detecting a slackening of the web as well as web breaks, the receiver and detector have in recent history been positioned on the same side of the web so that the system scans for continuous reflection of the emitted light from the web surface. Properly adjusted, systems of this type can be responsive to both breaks and slackenings of the web which cause the web to move outside of a predetermined zone defined by the scan head optics.
Systems of the foregoing type have had several limitations, however. Since they are dependent upon the reflection of light from the web, they are sensitive to variations in the web position which alter the angle of light reflection between the transmitter and receiver. Since this angle has been fixed in previous systems, there is only one optimum position for the web to be traveling in. The range of permissible variation from that position is quite small. Second, prior reflection-type scan systems have been extremely sensitive to variations in the web color and texture, since both of these factors affect the reflectivity of the surface being monitored. Of course, a dark colored web obviously absorbs more light and reflects less. Hence, its detectability is more limited than is the detectability of a lighter colored shiny web. However, if the system optics and sensitivity of the prior systems are adjusted to accommodate the dark, coarse web, problems may arise in the presence of a white shiny web in that so much light is reflected at so many angles that the optics and electronics of the receiver are essentially saturated and insensitive to the minor light variations that occur when the web begins to separate or tear.
Some prior art systems have attempted to solve the foregoing problems by electrical adjustments of the gain and sensitivity in the receiver circuitry. However, such adjustments are difficult to implement due to the inherent nonlinearities between system response to various colors and web locations. While the system may be capable of detecting both white and black webs at one web location, its ability to handle both color variations will be nonexistent at other web locations due to the angular differences of light reflection.